Premature treatment dropout is a problem for many families seeking mental health services for their children. Research is currently limited in identifying factors that increase the likelihood of dropout in families of school-aged children with Attention-Deficit/Hyperactivity Disorder (ADHD). Thus, the goal of the current study was to examine different child, parental, and family factors that predict dropout for families of children with ADHD at the initial assessment and during treatment. Participants included 73 families who sought treatment services at a university-based ADHD clinic. Variables found to be related to assessment or treatment dropout included ethnicity, ADHD medication status, severity of behavioral problems, and single parent families. In addition, single parent families strongly predicted assessment and treatment dropout. This provides support for identifying child and parental factors early in the evaluation process in order to modify interventions to reduce the risk of families discontinuing treatment before obtaining the benefits of a behavioral intervention. (Contains 8 tables and 1 note.)